A helicopter having a single main rotor must employ some means to counteract the torque produced on the fuselage of the helicopter by the engine turning the rotor blades. That is, the forces exerted by the engine against the rotor blades will, if not counteracted, cause the fuselage of the helicopter to rotate in a direction opposite to that of the main rotor. If allowed to continue, such rotation will quickly cause the helicopter to loose control and thereafter most likely result in a crash.
As shown in FIG. 1, traditionally, to counteract the engine torque, helicopters with single main rotors 20 have used a smaller tail rotor 40 set at the end of a tail boom 30 and positioned in a plane 42 generally perpendicular to the plane 22 of the main rotor 20. The tail rotor provides a sideward force (ft) which when applied to the moment arm (l) of the tail boom produces a counter-torque or moment (Mt) which opposes the torque (Me) produced by the engine driving the main rotor 20. As such, the counter-torque (Mt) allows the fuselage 10 of the helicopter to be maintained in a constant orientation in a yaw direction. Tail rotors also are normally designed to be able to vary the torque they produce by varying the pitch of the blades 44. This allows the fuselage 10 to be yawed in either direction to provide directional control.
Although clearly functional, this traditional design has many inherent disadvantages. One such disadvantage is that the tail rotor 40 must be powered by the engine, thus reducing the performance of the helicopter. Another disadvantage is that as the fuselage 10 is rotated to one side or the other, relative to the airflow, the tail boom 30 is forced into the airflow and significant forces are exerted on the tail boom 30 by the combination of this airflow and the main rotor downwash. This is shown in FIG. 2. The side forces (fs) cause a moment (Ms) to be applied to the helicopter, which must be compensated for by the pilot. This required pilot compensation is particularly troublesome in combat helicopters which must be able to quickly and accurately position the fuselage to perform offensive and defensive maneuvers. In addition, when the side moment (Ms) is in the same direction as the engine torque (Me), as shown in FIG. 2, the side moment (Ms) adds to the engine torque (Me), then additional power must be drawn by the tail rotor 40 from the engine and further reduces the yaw control available to the pilot. This reduces the overall performance of the helicopter.
A variety of approaches have been taken in the past to attempt to alleviate these problems. One approach has involved the use of various devices placed on the tail boom (e.g. strakes) to distort the air flow over the tail boom and as such reduce the resultant forces. Examples of this approach include those set forth in U.S. Pat. No. 4,708,305, entitled HELICOPTER ANTI-TORQUE SYSTEM USING FUSELAGE STRAKES, issued Nov. 24, 1987 to Kelley et. al., which is hereby incorporated by reference and U.S. Pat. No. 5,209,430, entitled HELICOPTER LOW-SPEED YAW CONTROL, issued May 11, 1993 to Wilson et. al., which is hereby incorporated by reference. Although the location, size and shape of the strakes differ, in each of these two devices the strakes are placed along the length of the tail boom, such that any airflow around (as opposed to along in a longitudinal direction) the tail boom will be spoiled. That is, these devices interrupt the air flow resulting in flow separation. Once separated the flow no longer produces as much side force as when the flow is attached.
Another approach to reducing tail boom loads it embodied in the NOTAR concept. NOTAR, or NO TAil Rotor, is an active system which uses in place of a tail rotor, forced air ejected through longitudinal slots along the boom to control the circulation about the boom and thus the resultant aerodynamic loads created primarily by the down wash of the main rotor. The NOTAR design controls the yaw of the fuselage by increasing or decreasing the lateral aerodynamic loads on the tail boom. Likewise, other concepts have used changes in the geometry of the tail boom to attempt to reduce and control the lateral aerodynamic loads. Essentially, such a design shapes the boom so that the airflow, which primarily from the main rotor downwash, produces lateral lifting loads on the tail boom.
However, these prior approaches have many inherent disadvantages. For instance, the strake and geometry approaches tend to provide insufficient reductions in both the static and dynamic forces exerted on the tail boom. Further, these approaches may increase the download on the boom associated with the rotor downwash, decreasing performance. Being an active system, the NOTAR concept requires significant power to be drawn from the helicopter""s engine to eject enough air from the tail boom. The power is used to power a fan which compresses air which is then passed through and out of the tail boom at a longitudinal slot and tip jet.
Therefore, a need exists for a device which provides sufficiently reduced loads on the tail boom such that improved maneuvering of the helicopter can be obtained. To retain acceptable performance of the helicopter, the device must be able to provide the reduced loading without drawing power from the engine.
The apparatus of the present invention provides a flight vehicle tail assembly having an exterior surface, one or more first or high pressure vents in the exterior surface, one or more second or low pressure vents in the exterior surface, and an air passage connecting the first vent(s) to the second vent(s) allowing air to flow therebetween. The first vent(s) are located at, or at least near, a high air pressure area acting on the exterior surface during a range of predefined flight conditions. Further, the second vent(s) are located at, or at least near, a low air pressure area acting on the exterior surface during the predefined flight conditions. Preferably, the vents run longitudinally along the length of the tail assembly at, or at least near, their respective pressure areas. As such, during the predefined flight conditions, adverse loads on the tail assembly can be reduced by venting air from the high pressure area, through the tail assembly, to the low pressure area.
The first vent(s) can include an air permeable cover positioned to cover at least a portion of the first vent(s) Likewise, the second vent(s) can also include an air permeable cover positioned to cover at least a portion of the second vent(s) Preferably, the air permeable covers are sized to cover the entire openings of each the first and second vent(s).
The tail assembly can include one or more first or high pressure vent doors and/or one or more second or low pressure vent doors. The vent door(s) are mounted to the tail assembly such that in a closed position they can cover the vent(s). Specifically, the first vent door(s) have a first door closed position. With the first vent door(s) in the first door closed position the first vent door(s) cover at least a portion of the first vent(s). Similarly, the second vent door(s) have a second door closed position. With the second vent door(s) in the second door closed position the second vent door(s) cover at least a portion of the second vent(s). Preferably, with these vent doors in their closed positions, their respective vent openings will be covered, and in open positions they will allow air to flow through the vents. The vent doors can include a spring which urges them into the closed position or an actuator which moves the door(s) into either the open or closed positions.
The air passage can be a cavity in the tail assembly defined by the structure of the tail assembly (e.g. skin, support structure, and the like). But, in at least one embodiment, the air passage is defined by a duct mounted within the tail assembly.
The tail assembly can also include one or more strake(s) mounted to the exterior surface and positioned to disrupt airflow across the exterior surface. The strake(s) can be positioned adjacent to either or both the first and second vents to facilitate in directing air in or out, as the case may be, of the vents. Further, with the vent door(s) set to open outward, the vent door(s) also function as strake(s).
The method of the present invention alleviates adverse aerodynamic loads on the flight vehicle tail assembly. The method includes the steps of: receiving air through one or more first or high pressure vents in the exterior surface, passing the air through the tail assembly from the first vent(s) to one or more second or low pressure vent(s) in the exterior surface, and ejecting the air out of the tail assembly at the second vent(s).